1. Field of the Invention
The present invention relates to a color cathode ray tube and a method of manufacturing a shadow mask used in the color cathode ray tube.
2. Description of the Related Art
A shadow mask type color cathode ray tube comprises a glass envelope having a face panel, a funnel and a neck; a phosphor screen on which a plurality of phosphor dots or stripes are regularly arranged and which is formed on an inner surface of the face panel; and an electron gun disposed in the neck portion of the envelope to emit plural electron beams to the phosphor screen. Further, a shadow mask having a large number of regularly arranged electron beam apertures is disposed in the envelope, more closely opposing to the phosphor screen between the phosphor screen and the electron gun.
The shadow mask based on the principle of parallax is one of significant components which has a function of allowing plural electron beams shot from the electron gun to pass therethrough to correctly land on their geometrically corresponding phosphor dots or stripes, and it is called a color selection electrode.
Each electron beam coming to the peripheral portion of the shadow mask has a certain angle relative to the tube axis of the cathode ray tube. Each electron beam aperture, therefore, has such a specific shape that allows the electron beam to easily pass therethrough. In short, each electron beam aperture of the shadow mask has a larger sectional area on the phosphor screen side of the shadow mask, as compared with that on the electron gun side thereof. Usually, this part of the aperture which is on the phosphor screen side is called as a large opening and that part thereof which is on the electron gun side is called as a small opening to distinguish them from the other in sectional area.
The shadow masks are generally grouped to those having circular electron beam apertures and those having rectangular ones. The former are usually used in display tubes that display characters and figures while the latter in home-used tubes such as television tubes.
Recently, the display tubes are more often used as display units in personal and office computers or in various kinds of OA terminal equipment. Therefore, an image whose resolution is enhanced to a greater extent and which less reflects external light and has less distortion is demanded from the viewpoint of human technology. In order to meet these demands, the color cathode ray tube having a flatter face panel has been provided.
When the face panel is made flatter, the shadow mask which is same in shape as the face panel must also be made flatter and have a larger radius of curvature. In the flattened shadow mask, however, the angle of the electron beam which enters into its corresponding electron beam aperture becomes larger relative to the normal of the mask, as compared with that in the conventional shadow mask having a small radius of curvature. Needless to say, the angle of incidence of the electron beam becomes larger at the peripheral portion of the mask than at the center portion thereof, and part of the electron beam incident on the peripheral portion, therefore, collides against the aperture edge or aperture wall at a higher rate. When the electron beam collides against the aperture edge or aperture wall, the shape of the electron beam spot formed on the phosphor screen is distorted or so-called beam omissions are caused, thereby degrading the luminance or the uniformity of color purity. In addition, the contrast is also degraded because unintended phosphor dots are made luminous by electron beams reflected by the aperture edges and aperture walls.
The problem of beam spot distortion is more liable to be caused as the pitch of the electron beam apertures in the shadow mask becomes smaller and the shadow mask is made becomes thicker. In addition, it is more remarkable as the angle of incidence of the electron beam relative to the electron beam aperture becomes larger, as seen in the shadow mask which is made flatter and which has a larger radius of curvature. The quality of the color cathode ray tube is thus degraded.
Further, when the curvature radius of the shadow mask becomes larger, the tension strength of the mask is lowered to a greater extent, as compared with that of the conventional shadow mask whose curvature radius is small. The shadow mask, therefore, is more easily deformed by impacts added when the color cathode ray tube is being manufactured, transported and incorporated into the television set. That part of the shadow mask which is thus deformed cannot have a predetermined distance relative to the phosphor screen. Color shift is thus more easily caused and the quality reliability of the color cathode ray tube cannot be guaranteed. When the shadow mask is too excessively deformed, it has a complete partial color shift and it must be regarded as a defect one.
As means for preventing the beam spot distortion or beam omissions, it is imagined that the large opening of the electron beam aperture which opens on the phosphor screen side face of the shadow mask has a larger dimension. In this case, however, the large opening of the aperture must be etched larger in amount when it is formed in the shadow mask. The mechanical strength of the shadow mask is thus lowered, thereby reducing the tension strength thereof to a greater extent and causing the mask to be more easily deformed after it is press-formed.
In the shadow mask in which the electron beam apertures are regularly arranged at a small pitch to attain a high resolution, however, it is difficult that the wall of each aperture is so tilted as to enable the electron beam to completely pass therethrough even when the dimension of the large opening of each aperture is made so large that large openings of the adjacent electron beam apertures can be contacted with each other at their rims or edges on the surface of the shadow mask.
In order to solve this problems, Jpn. Pat. Appln. KOKOKU Publication No. Sho 47-7670 has proposed a so-called off-center mask in which the aperture center of the large opening of the electron beam aperture in the shadow mask is deviated from the aperture center of the small opening of the aperture in a direction in which the electron beam passes. This method of deviating the center axis of the large opening of the aperture from that of the small opening thereof to an extent needed is efficient for preventing a beam omission from being caused when the incident electron beam collides against the wall surface or edge of the large opening of the aperture. It is also efficient for preventing the mechanical strength of the mask from being reduced because the dimension of the large opening of the aperture can be kept small.
In the off-center mask, however, it is needed that the extent to which the center axis of the large opening of the aperture is deviated from that of the small opening thereof is made large to prevent the beam omission. When the electron beam aperture is viewed in the thickness direction of the shadow mask, therefore, its physical diameter becomes different in dimension from that of the beam spot formed on the phosphor screen by the electron beam which has actually passed through it. Further, the shape of the electron beam aperture formed at a boundary between the large and small openings of the aperture is not circular but deformed, and it is not stable accordingly. In the color cathode ray tube which is small in freedom of the electron beam landing area on the phosphor screen, therefore, degradation in the uniformity of color purity is more liable to be caused.
In order to make the off-center amount between the large and small openings of the aperture small and to tilt the wall surface of the large opening thereof to an extent needed, therefore, the dimension of the large opening must be made large to a limit although this limit depends upon the pitch of the apertures. In the shadow mask flattened and having a large radius of curvature, its tension strength becomes low after it is press-formed. As the dimension of the large opening of the aperture is set larger and larger, however, its mechanical strength becomes lower. This causes the shadow mask to be more often deformed.
When the thickness of the shadow mask is made large to increase its mechanical strength, it becomes difficult to control the etching by which each electron beam aperture is formed in it. Its quality is thus degraded. When it is made thick, the tilt of the wall surface defining the large opening of the aperture needed is also increased. The off-center amount must be therefore made large, thereby causing same problem.
As means for preventing beam omissions, it is imagined that the length from the boundary between the large and small openings of each electron beam aperture to that surface of the shadow mask which is on the side of the electron gun is made long and that the tilt of the wall of the large opening of each electron beam aperture needed is made small. In this case, however, the amount of the electron beam colliding against the wall surface of the small opening of the aperture is increased and the contrast is lowered by the electron beam reflected by this wall surface.